The present invention relates to a method and an integrated circuit for boosting an input voltage with the aid of switches and capacitors.
It is often desirable to construct equipment as small and light as possible, particularly portable electronic devices such as radios, telephones (mobiles), audio devices, dictaphones, hearing aids, watches, cameras, and so on. Several batteries in a device results in an undesirably large space requirement and a high weight. On the other hand, for physical reasons the modules, which are often constructed in the form of an integrated circuit like a memory or an amplifier, must demand a particular minimum voltage in order to guarantee their function. Besides, some integrated circuits need several different operating voltages, which cannot be drawn from a single battery without great expense.
An individual battery cell, which can have 1.2 to 1.5 volts as a rule, depending on the embodiment, often is insufficient for the above-described problem, especially since the battery voltage can drop further as a result of the charge loss of the battery.
This problem has been handled in the past by making the battery configuration smaller and then connecting several smaller batteries in series in order to generate a higher voltage (input voltage). The disadvantage of this is that a relatively large space is still required.
In order to generate a higher voltage from a low voltage, voltage converters with transformers or the like have-been used. But these function only with A.C. voltages and cannot be realized in battery-operated devices without additional outlay.
A pump circuit such as a charge pump is also known, with which the input voltage can be doubled for a reasonable expense. The charge pump is employed particularly in integrated circuits, whereby corresponding capacitors are charged by MOSFET transistors as switching transistors. The disadvantage of a single-step charge pump is that the voltage can be at most doubled.
U.S. Pat. No. 5,568,079 describes a method for boosting an input voltage and a corresponding integrated circuit with a two-stage charge pump. But the disadvantage of the configuration is that the output series pass transistor must have a relatively large width in order to make reliable switching possible.
It is accordingly an object of the invention to provide a method and an integrated circuit for boosting a voltage that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which it is possible to utilize a small output series pass transistor that is characterized by a high reliability.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for boosting an input voltage. The method includes impressing the input voltage on a two-stage charge pump during a charging phase resulting in a stored input voltage and transforming the stored input voltage into a boosted output voltage during a pumping phase. The boosted output voltage is charged by way of an output series pass transistor and tapped at an output capacitor. A level shifter of the two-stage charge pump is used for driving a bulk terminal of the output series pass transistor.
According to the invention, for the purpose of boosting the input voltage, the two-stage charge pump is provided with a level shifter with which a bulk terminal of an output series pass transistor is drivable. It is possible to drive the bulk terminal particularly in an isolated NFET transistor, because the bulk terminal is located in a separate trough that is isolated from the substrate. With this layout, it is possible to raise the bulk terminal of the output series pass transistor to the desired pump voltage, so that the output transistor can be switched to low impedance. Furthermore, a high reliability of the output series pass transistor is achieved, because the voltage of the source, drain and gate are reduced relative to the bulk. By driving the bulk terminal, the width of the output series pass transistor is reduced as the bulk effect is reduced.
The advantage of the inventive method for boosting an input voltage and the inventive integrated circuit is that, with the aid of a two-stage charge pump, the input voltage can be boosted to triple its value without further ado. It is particularly advantageous that, with the configuration, the voltage can be easily raised to substantially higher values, for instance to a triple value. When the method is applied to an integrated circuit, in particular, the voltage boost can be achieved without any appreciable additional outlay in the fabrication of the integrated circuit.
It is particularly advantageous that the two-stage charge pump has the same construction and/or utilizes the same drive signals as the single-stage charge pump. The layout associated with the utilization of the two-stage charge pump is thus substantially simplified.
In order to minimize the ON-resistance of the series pass transistor, this is driven with an elevated gate voltage, whereby the gate voltage is expediently selected larger than the source voltage. It is then no longer necessary to enlarge the output series pass transistor.
It is also particularly advantageous that the output voltage is utilized for generating the gate voltage in order to open the output series pass transistor. The gate voltage is raised therein to double the output voltage.
It is also advantageous that in the first phase the two capacitors are charged to the input voltage simultaneously. This is possible because the same drive signals are utilized.
With the chosen two-stage construction of the charge pump, the output voltage can be transformed upward directly to the desired value. In contrast, with two individual charge pumps that are connected in series, the output voltage would depend to a large extent on the generated voltage of the first charge pump.
In accordance with an added mode of the invention, there is the step of applying the boosted output voltage at the bulk terminal of the output Series pass transistor by way of the level shifter.
In accordance with an additional mode of the invention, there is the step of driving a gate terminal of the output series pass transistor using an additional level shifter and a capacitor of the two-stage charge pump.
In accordance with a further mode of the invention, there is the step of applying double the value of the boosted output voltage at the gate terminal of the output series pass transistor.
In accordance with another mode of the invention, there is the step of using the input voltage from an electronic circuit.
In the integrated circuit, it is advantageous that the switches, i.e. the switching transistors and/or the capacitors, are disposed at least partly in isolated wells of the substrate. They are thus separated from neighboring elements with respect to potential and can be wired like discrete components.
Field effect transistors that are constructed as isolated NFET transistors are preferably utilized as the switches. They are situated in an isolated well and can be controlled via the bulk terminal, so that the bulk can be higher than 0 volts.
The circuit layout is simplified by driving the gate terminal of a transistor with a level shifter. In particular, additional capacitors that would otherwise take up a relatively large amount of chip space can be spared this way. Besides this, the bulk can be driven, and consequently a smaller transistor width is sufficient.
With the foregoing and other objects in view there is provided, in accordance with the invention, an integrated circuit. The integrated circuit contains a two-stage charge pump for transforming an input voltage into a boosted output voltage. The two-stage charge pump includes an output capacitor, and an output series pass transistor connected to and charging the output capacitor and the boosted output voltage being available at the output capacitor. The output series pass transistor has a bulk terminal. At least one level shifter is provided and has an output connected to the bulk terminal of the output series pass transistor for driving the bulk terminal.
In accordance with an added feature of the invention, the level shifter applies a value of the boosted output voltage to the bulk terminal of the output series pass transistor.
In accordance with an additional feature of the invention, the output series pass transistor has a gate terminal, the two-stage charge pump has a capacitor, and the two-stage charge pump includes an additional level shifter having an output coupled to the gate terminal of the output series pass transistor through the capacitor for driving the gate terminal.
In accordance with a further feature of the invention, the additional level shifter applies double the value of the boosted output voltage to the gate terminal of the output series pass transistor through the capacitor.
In accordance with another feature of the invention, a substrate having isolated wells formed therein is provided. The two-stage charge pump has capacitors disposed at least partly in the isolated wells of the substrate. In addition, the two-stage charge pump has switches, and the switches and the output series pass transistor are disposed at least partly in the isolated wells of the substrate.
In accordance with concomitant feature of the invention, the two-stage charge pump has switches constructed as switching transistors. More specifically, the switching transistors are field effect transistors or N-type FETs.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an integrated circuit for boosting a voltage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.